1. Field of the Invention
This invention relates to a method for determining the concentration of specific ions in solution. More specifically the invention provides a method of making determinations of ionic concentrations solely by electrical measurements of specific conductivity and pH without any necessity of removing ions from solution.
2. Description of the Prior Art
A number of methods are already known in the art for measuring the concentration or specific ions in solution. For example, the traditional methods employed in quantitative analysis are suitable. One such method involves the introduction into the sample solution of a reagent which is capable of forming a precipitate with the ion whose concentration is to be measured. The precipitate is separated from the solution, dried, weighed, and then the amount of the ion of interest is calculated based on the amount of precipitate formed. This method is cumbersome and time consuming and requires essentially non-portable equipment such as accurate weighing apparatus and an oven. Moreover, the method requires some skill on the part of the operator.
It is also known that the determination of the concentration of many ions can be made by the use of commercially available potentometric electro-chemical electrodes. Such electrodes yield a continuous-signal output voltage having a simple logarithmic relation to ion activity, and a sufficiently fast response. This logarithmic relation is the well-known Nernst equation. However, the output voltage which is observed is not necessarily simply related to concentration for a number of reasons. First, the activity of a given ion in a solution is affected by the background activity of other ions, i.e., the total ionic strength of the solution. As the strength of the latter increases, generally one expect that the activity of a given ion apparently decreases. Also, often ions are complex and in such a case, the measured activity differs markedly from the concentration. For example, an electrode sensitive to fluoride ion activity simply does not given an accurate measurement of the fluoride concentration, for example in sea water where there is an excess of magnesium ion present because fluoride can be complexed by magnesium.
It is desirable in many instances to show the concentration of a species of ions in solution rather than its activity. A method and an apparatus for this purpose is disclosed in U.S. Pat. No. 3,591,481 issued July 6, 1971 to Rieseman. In this method concentration measurements are effected by relating the electrode potential resulting from an unknown or sample solution with the electrode potential from a mixture of that sample solution with a standard solution of known concentration of the ion of interest or a strong complexing or precipitating agent for the ion of interest. The method of obtaining a measurement requires two essential steps. First, the electrode sensitive to the ion of interest is immersed into a specific volume (e.g., 50 ml.) of the sample solution containing an unknown amount of the ion of interest; the potential developed then causes a deflection of a meter movement in accordance with the Nernst equation. The meter movement is then adjusted to a null point and a small known volume (e.g., 1 ml.) of a standard solution containing a known concentration of the ion of interest (or a strong complexing or precipitating agent therefor) is added to the sample. The addition of the standard solution will change the total so that a new potential will arise which causes the meter movement to deflect away from the null point by some value .DELTA.E. Since the concentration of the standard solution is predetermined, the concentration of the sample will appear as a reading on the meter scale or base expressed typically in moles per liter based on the value of .DELTA.E.
The method of this invention provides an alternative to the method in the foregoing patent and differs essentially therefrom in that an excess of the complexing or precipitating agent in the standard (reagent) solution is used, that the concentration of the complexing or precipitating agent need not be predetermined and that the measurements made are of pH and specific conductivity rather than electrode potential as in the reference.